1. Field of the Invention
The present invention relates to methods for fabrication of interdigitated back contact photovoltaic cells.
2. Description of the Related Technology
Interdigitated back contact (IBC) cells are photovoltaic cells having both emitter contacts and base contacts at the rear side of the cells, the emitter contacts and base contacts being interdigitated for separate collection of electrons and holes. Eliminating front side contacts avoids the need for making a trade-off between shading losses and series resistance. Fabrication processes for IBC cells often require several masking steps and alignment steps, because at the rear side of the cells there is a need for separating emitter regions and base regions and for separating and properly aligning emitter contacts and base contacts.
Methods have been proposed for reducing the complexity of process sequences for manufacturing IBC cells. For example, in “Realization of self-aligned back contact cells”, Electrochemical and Solid-State Letters, 11 (5), H114-H117, 2008, P. Papet et al describe a process sequence wherein only one lithography step is used and wherein no alignment steps are needed afterwards. Separation between emitter contacts and base contacts is obtained by chemical etching of grooves in a silicon substrate using a metal mask, wherein underetching of the mask leads to the formation of cantilevers, the cantilevers being sufficiently large (a few micrometers) to avoid short circuits between both contact types after metal deposition.
In “Super self-aligned technology for backside contact solar cells: a route to low cost and high efficiency”, IEEE PVSC 1990, P. Verlinden et al propose a method for fabricating backside contact photovoltaic cells, the method requiring only one photolithography step without alignment. Separation between n+ doped regions and p+ doped regions at the rear side of the cells is obtained by chemically etching grooves in the silicon substrate using an oxide mask and underetching the mask such that oxide cantilevers are formed at the sides of the grooves. The bottom and the sidewalls of the grooves are thermally oxidized to provide a passivation layer, and a silicon nitride layer is deposited on top of the oxide layer. Next, using anisotropic Reactive Ion Etching, and using the cantilevers as a mask, a window is opened in the oxide layer and the nitride layer at the bottom of the grooves. The silicon oxide layer and the silicon nitride layer remain at the sidewalls of the grooves. A boron predeposition is then performed and both doping regions (n+ emitter region and p+ region in the base) are diffused in a single high temperature step. In order to provide a separation between n+ doped regions and p+ doped regions, the silicon oxide/silicon nitride stack present at the sidewalls of the grooves is used as a masking layer. For forming emitter contacts and base contacts, an Al layer is deposited by vacuum evaporation which features a very poor step coverage, such that the presence of the cantilevers leads to a separation between emitter contacts and base contacts.